This invention relates to quadrant arrays of photodetectors and in particular to avalanche photodiode (APD) arrays in which the transition regions between quadrant array elements are bridged by a reflective optical divider which causes radiation to be incident on photosensitive surfaces at large angles from normal incidence and in which the surfaces of the photosensitive region of the array are coated with an antireflection coating material.
Modern optical acquisition and tracking applications often require the use of a plurality of photodetectors in order to steer the boresight of the optical system toward the target. An APD array such as a quadrant avalanche photodiode (QAPD) having four photosensitive elements arranged in quadrants may be used in place of four separate photodetectors with image dividing optics to reduce the size and weight of the photodetector assembly.
Two different types of QAPD's have been developed. The first type of QAPD was a P-side divided QAPD which had a P-type light entry side contact divided into quadrants with higher resistance Pi-type material in the transition regions between the quadrants. The P-side divided QAPD suffered from several deficiencies, however, including crosstalk between the quadrants due to undepleted material in the transition region between quadrants. Additionally, the P-side divided QAPD had a significant amount of Johnson noise current due to the transition region leakage resistance which was detrimental in low bandwidth tracking applications.
Subsequently, a QAPD divided on the N-type Junction side (J-QAPD) was developed. The J-QAPD had no leakage resistance between the quadrants so that it performed better than the P-side divided QAPD in all areas except for its lack of response to signals received in the transition region between quadrants. Thus, while the J-QAPD lacked crosstalk and Johnson noise, the J-QAPD suffered from a dead zone in which carriers photogenerated by the incident light in the transition region between photosensitive quadrants were lost and did not contribute to the photodiode's output.
Numerous attempts have been made to provide an optical divider configuration which could be mounted upon the J-QAPD to bridge the dead zone and deflect light impinging upon the dead zone into a sensitive region of a quadrant, and thus provide a small effective transition region. In one approach, a mirrored prism structure with an acute angle and a very fine edge has been utilized to straddle the dead zone and deflect light into the appropriate quadrant. Typically, this mirror prism structure is comprised of silicon which has been photolithographically masked and etched to form the prism structure and coated with chrome and gold to provide a reflective surface for the impinging light rays.
The Etched Silicon Divider (ESD) forms a prism with an apex angle of 70.6 degrees which deflects the light rays at too shallow an angle for all of them to strike the underlying J-QAPD within its sensitive region. For example, rays which strike near the apex of the divider pass above the photosensitive surface and strike outside the sensitive diameter, especially near the edges. A back reflector was incorporated into the design of the ESD in order to capture the rays directed beyond the sensitive region and to deflect them downwards towards a sensitive region of the J-QAPD. While this ESD design incorporating the back reflector greatly improved the performance of the J-QAPD/ESD combination, this design still exhibited characteristic losses near the base of the reflecting prism surfaces and especially near its center along the 45 degree diagonal lines which divided each quadrant. These losses corresponded to impinging rays which hit near the base of the reflecting prism surfaces. The rays reflecting from the mirrored prism surfaces strike the sensitive region at a variety of angles and are expected to be absorbed by the photodetector or reflected toward the back reflector and subsequently photodetected. However, in prior art devices, the rays striking the sensitive region near the base of the mirrored prisms suffered unexplained losses and were thus not useful to many applications which require that all incident signal contribute to the measurement.
The impinging rays which hit near the base of the mirrored prism close to the diagonals are reflected from two intersecting faces of the mirrored wedge shaped dividers prior to striking a photosensitive region at an angle typically greater than 80 degrees form the normal. The impinging rays which hit near the base of the mirrored prism away from the center are reflected once and subsequently impinge upon the photosensitive region at an angle typically greater than 70 degrees from the normal. Rays which are incident on the mirrored prism and take one or two reflections before striking the photosensitive region arrive at very steep angles of incidence. The grazing angle of incidence at which the ray strikes the photosensitive region is typically a very large angle as measured form the normal to the surface of the sensitive region such as greater than 70 degrees. It is desirable that the rays striking the sensitive region at such a large angle would be photodetected (absorbed) or reflected toward the back reflector and subsequently photodetected. However, in prior art devices, the ray striking the sensitive region at such a large angle from the surface's normal lost a substantial portion of its energy passing through the antireflection coating upon the sensitive region and thus contributed very little to the desired output signal.
A large loss of energy by the ray striking the sensitive region at a grazing angle of incidence is typically due to the heat produced during the rays travel through the antireflection coating deposited upon the photosensitive surface. This antireflection coating has typically been silicon monoxide (SiO) which absorbs a large portion of the ray's energy and dissipates it as heat when the ray strikes the antireflection coating at such an angle so that a long path length is travelled prior to its arrival at the sensitive region. While silicon monoxide is an inefficent coating in this instance due to its inherent optical loss, silicon monoxide is generally an excellent choice for the antireflection coating material since the losses introduced are small for light rays entering substantially perpendicular to the surface of the J-QAPD.
That portion of the ray's energy reflected from the coated photodetector surface reaches the back reflector which redirects it toward a photosensitive region, however, the amount of energy delivered to the photosensitive region by this redirected ray is typically very small due to the previous dissipation of energy within the antireflection coating on the photosensitive region.
Therefore, a J-QAPD/ESD combination would be desirable which exhibited no characteristic losses along the base of the prism neither near its center along the 45 degree diagonal lines dividing the quadrants nor away from its center so that impinging light rays striking the lower portion of any prisms could effectively contribute to the output signal of the J-QAPD. Furthermore, it would be desirable for a J-QAPD to be developed having antireflection coatings comprised of a material exhibiting low loss so that substantially all the energy of the impinging light ray would reach the photosensitive region of the J-QAPD to provide a useful output